The structure, function and expression of keratin intermediate filaments of human skin, and the related intermediate filaments of other cell types, are being investigated. While the roles of these proteins in many genetic diseases are now well understood, further structural studies are necessary to develop rational approaches to therapy. We have constructed a series of synthetic peptides of sequence corresponding to keratin chains which are involved in the important overlap regions in assembled filaments, for biophysical structural studies, and for use for X-ray crystallography. Several of these synthetic peptides have been injected into living cells to explore their dynamic behavior. Some of these function as very specific reagents for the disruption of all types of intermediate filaments, or of only keratin filaments. In order to explore the functions of end domain sequences, we have used the two yeast hybrid system. We have expressed keratins 5 and 14 to explore their structures and functions in vivo. In other studies, we have determined that a portion of the V1 subdomain region of type II keratins is involved in crosslinking with other proteins to the cornified cell envelope of stratified squamous epithelia, a process which coordinates the structural organization of the keratin filament cytoskeleton with the cell periphery in terminally differentiated keratinocytes. Analyses of other crosslinks reveal that the keratins are attached to the desmoplakin component of desmosomes indirectly through a series of related intermediate filament associated proteins.